Lithography or offset printing is currently the dominant printing technology. The printing is typically performed by a printing form/plate that has a specially prepared/treated surface, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water or special solution, will repel the ink. For a positive printing plate, the areas that accept ink form the printing image areas and the water accepting (ink-repelling) areas form the background areas. For a negative printing plate, the plate imaging conditions are reversed. The specially prepared/treated plate surface typically comprises an anodized aluminum surface, coated by a thin layer of ink receptive photopolymer that hardens when exposed to an image-wise radiation (the radiation that creates the image on the plate). In a subsequent development stage, the non-exposed areas are washed out, exposing the water accepting anodized aluminum surface, and leaving islands of hardened, ink receptive polymer. This process results in a plate that provides excellent image quality and sustains long mechanical wear and tear, typical for long printing runs.
Use of this type of printing plates implies use of water on the printing press. The complicated relationship between the amounts of water and the amount of ink requires significant skills from the press operator. Another disadvantage of these printing plates is that the development or processing involves the use of chemicals and/or solvents, the disposal of which requires certain care and control. The plates are typically sensitive to UV radiation and the scarcity of digital UV radiation sources complicates their exposure by digital exposure means such as platesetters.
To overcome this latter obstacle, development of so-called thermal offset printing plates was undertaken, for which a large variety of IR radiation sources exist. These plates have recently been commercialiazed by a number of companies. A typical example of a thermal offset printing plate is Kodak Polychrome Direct Image Thermal print plate, commercially available from Kodak Polychrome Graphics, Norwalk, Conn., U.S.A.
Further efforts to reduce or eliminate plate processing have resulted in so called ablation type plates, disclosed in U.S. Pat. Nos. 5,339,737, 5,551,341 and 5,632,204 all to Lewis et al. These patents disclose a lithographic printing plate flat is transformable by laser-discharge techniques, so as to change its affinity for ink. The plate is of the ablation type where a layer or number of layers, the outermost of which is typically made of silicone, are ablated by powerful laser radiation. The outer silicone layer is ink-repelling and the layer underlying the silicone layer is ink-receptive. Laser output typically ablates one or more plate layers, in either case resulting in an image-wise pattern of features on the plate. The plate may include additional layers tat facilitate the ablation process or help in laminating the ablating layer to the aluminum substrate. The plate preparation process may be performed on the press and reduces press make-ready time and potential damage to the plate. Such plate is commercially available from Presstek Inc., Hudson, N.H., U.S.A. and used in a printing press QM-46 DI disclosed in U.S. Pat. No. 5,339, 737 and commercially available from Heidelberg Druckmachinen AG, Heidelberg, Germany.
The ablation process, however, is accompanied by the generation of a large amount of imaging debris and it is necessary to clean the plates to remove this debris before the printing process is commenced. The debris interferes with the laser radiation, by depositing on the focusing lens as an aerosol or mist of fine particles that block the laser radiation transmission. This creates a need for frequent cleaning of the optics and of the exposure compartment.
Digital plates for the conventional "wet" process are imaged by ablating only the top layer (see Presstek "Gold" plate). The ablation products of this process are in the form of air-borne dust and need to be collected by vacuum.
U.S. Pat. No. 5,755,158 to Wolfe et al. discloses another cleaning apparatus for lithographic printing plates, that includes a rotating elastomeric roller that contacts imaged plates, which are typically (although not necessarily) carried on a rotary cylinder, at a velocity different from the velocity (if any) of the plate. The roller may spin in the direction of or opposite to, that of the cylinder and at substantially different speed. Typically, the apparatus is mounted proximate to the cylinder, circumferentially adjacent to the imaging system, and is retractable, so as to be selectively engaged when imaging is complete. The apparatus may include, in addition to, or in lieu of the elastomeric roller, a second retractable cleaning member for rubbing the imaged plate with a cleaning fluid. The second cleaning member may be an elongated cartridge, having an absorbent towel exposed along one face thereof. A cleaning fluid is dispensed onto the towel by, for example, a spraying device. The cartridge is then extended to urge the towel against the printing plate as it rotates. It is clear that such a solution adds cost to the printing equipment, increases its operating complexity and the make-ready time and, accordingly, the cost of printing.
A further effort to simplify the plate cleaning process is disclosed in U.S. Pat. No. 5,807,658 to Ellis et al. The patent discloses a self-cleaning, abrasion-resistant, laser-imageable lithographic printing construction. The construction is a wet lithographic printing plate that includes a protective layer to provide protection against handling and environmental damage, extends plate shelf life, and entrains debris generated by ablation. The protective layer washes away during the printing make-ready process, effectively cleaning the plate and disappearing without the need for a separate removal process. This method, although simplifying the plate usage, does not make the plate ready for printing immediately following the imaging stage. It also adds the cost of the washable layer to the plate and the washing facility to the press. This increases the plate preparation process complexity and the press make-ready time and accordingly the cost of printing. The need for additional, post imaging operations complicates the use of the plate in computer-to-plate devices and on-press digital imaging systems.
Different means have been tried to protect the exposure optics and plate surface from ablation debris. Typically, these are mechanical shutters and baffles that somehow absorb part of the flying debris. Vacuum, or a directed airflow through the gap between the exposure head and the plate, may also assist in the debris evacuation procedure. This, however, may be effective only in cases where all of the protective or oleophobic silicone layer particles are pulled off the plate and are airborne. Should some particles remain attached to the substrate, the vacuum assistance is of no use. Further, both the airflow and the vacuum create vortices that cause debris deposition on other parts of the system.
Exposure of polyester-based plates or films consisting of transparent substrate can be performed through the substrate. This can help in protecting the optics, but does not solve the plate/film-cleaning problem, as the ablation debris continues to be deposited on the film/plate surface and surroundig mechanical parts. This complicates the use of ablation plates and makes them less suitable for on-press imaging.